In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display panel or the like, a pattern is made by irradiating a ultraviolet light to a semiconductor wafer or a glass plate for liquid crystal, but if a dust particle is sticking to a photomask used in this stage, the dust particle obstructs the light or reflects it, causing deformation, short circuit or the like in the pattern being transferred, and such phenomena lead to lowering of the quality of the end products.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is yet difficult to keep the photomask clean all the time. Hence, in practice, the exposure light irradiation is conducted only after a surface of the photomask is sheltered by a pellicle as a dust fender. Under such circumstances, foreign particles do not directly adhere to the surface of the photomask, but only onto the pellicle membrane, and thus by setting a photo focus on a lithography pattern on the photomask at the time of lithographing, the foreign particles on the pellicle membrane fail to transfer their shadows onto the photomask and thus no longer become a problem to the image transfer performance.
In general, a pellicle is built up of a pellicle frame, which is an endless frame bar usually made of aluminum, a stainless steel, or the like, and a transparent pellicle membrane usually made of cellulose nitrate, cellulose acetate, a fluorine-containing polymer or the like which transmit light well; this pellicle membrane is attached via dried solution or adhesive to one of the two annular faces (hereinafter referred to as “membrane-side face”) of the pellicle frame. On the other one of the two annular faces of the frame (hereinafter referred to as “mask-side face”) is laid an agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like for attaching the pellicle frame to the photomask, and over this agglutinant layer is laid a releasable liner (separator) for protecting the agglutinant layer.
In recent years, owing to the increased refinement of the exposure light pattern, the problem of the deformation of the photomask caused by pellicle adhesion to it has become more focused. When the photomask and pellicle frame are coupled together via the agglutinant layer, the form of the pellicle frame affects that of the photomask, and thus the pattern described on the surface of the photomask is deformed from the original form.
Various countermeasures have been proposed to solve this problem. For example, IP Publication 1 teaches that by controlling the flatness of the mask-side face of the pellicle frame to 30 micrometers or smaller and the membrane-side face thereof to 15 micrometers or smaller, it is possible to minimize the mask deformation.
Then, in IP Publication 2, it is described that by regulating the thickness and elasticity of the agglutinant layer of the pellicle frame to certain ranges of values, the irregularity in the mask-side face of the pellicle frame is absorbed in the agglutinant layer so that the smoothness of the mask surface is not substantially affected.
IP Publication 3 teaches that by using a soft gel composition as the pellicle agglutinant it is possible to reduce the deformation of the mask, which is caused when the pellicle is adhered to the mask or the like.
Further, IP Publication 4 says that by restricting the stickiness of the agglutinant layer to a low range of 1 N/m through 100 N/m, it is possible to restrict the deformation of the mask caused by the adhesion of the pellicle.
However, these proposals are not proved to be sufficient countermeasures for mitigating the deforming effect of the pellicle frame to the photomask—hence the problem is far from being solved. For example, in the case wherein a soft agglutinant layer is used, when the pellicle is detached from the photomask, traces of the agglutinant layer tend to remain on the surface of the photomask, and their removal and re-cleaning of the frame would be no little problem.
Also, even if the flatness of the agglutinant layer is improved, there remains a problem in that unless the frame is adhered to the photomask under a large pressure, an air pool is formed between the agglutinant layer and the photomask so that the adhesion between them is weakened and there may be made an air leak passage across the pellicle frame bar, and thus the reliability is lost. On the other hand, if, in order to solve this problem, the frame is adhered to the mask under a larger pressure, the extent to which the photomask is affected by the form of the pellicle frame increases, and the resultant deformation of the photomask pattern is enhanced.
As such, the fact remains true that there has not yet been proposed a pellicle which is so contrived that the deformation imparted to the photomask by the pellicle as the latter is adhered to the former is sufficiently suppressed and at the same time the pellicle is easily removable from the photomask so that the reliability during the use is high.